A variety of products comprise glass bonded to metal or metal oxides in a composite structure. A particularly useful example is a magnetic recording and reading head. Such a head comprises, as is conventional, a ferromagnetic core structure containing a gap at the recording and reading site that is accurately joined by glass bonded to the cores. Such cores either comprise solid ferrite or they are ferrite provided with special plates forming the gapping structure. The plates are a composite of metal alloy wafers laminated with alternating layers of a bonding composition, usually epoxy polymers, the composite being shaped and attached as tips to the cores of the head. In the past, it has not been possible to bond such metal alloy structure with glass in a reliable manner. Glasses would have advantages over polymer adhesives, if they could be used. Such advantages include much higher mechanical modulus and a greater degree of thermal stability and hydrophobicity, which improve the dimensional stability of the metal heads.
Several problems have been encountered in the making of bonded metal or metal oxide and glass products. One problem has been to provide a glass that has a coefficient of thermal expansion that substantially matches that of the metal alloy or metal oxide. Failure to match the coefficient creates thermal stress under varying temperature conditions and eventual cracking of the glass.
Another problem has been that certain glass ingredients are highly volatile, and create bubbles as the glass solidifies. Such bubbles in a magnetic head degrade the integrity of the head structure. If located at the tape contacting surface of the head, they may provide a sharp edge that causes damage to the magnetic tape.
Still another problem has been that certain ingredients useful in glass formulations to match the thermal expansion coefficients of the metal alloy or metal oxide, also render the glass susceptible to chemical attack such as by caustic solutions. Because magnetic heads require a cleaning step using a caustic solution after machining, susceptibility to chemical attack by such caustics cannot be tolerated. It has been suggested that significant amounts of Al.sub.2 O.sub.3, i.e., at least 5 mole %, will provide the resistance to attack from caustic solutions. The difficulty has been that such amounts of Al.sub.2 O.sub.3 do not readily dissolve into many useful glass networks.
U.S. Pat. No. 3,458,926 describes one approach to the problem wherein the glass comprises up to 5.7 mole % Al.sub.2 O.sub.3. In order to solubilize this amount of Al.sub.2 O.sub.3, excessive SiO.sub.2 has to be used as the glass former, namely 52.9 mole %. Such a large amount of SiO.sub.2 is disadvantageous because it imparts a coefficient of thermal expansion that is too low (70.4.times.10.sup.-7 .degree.C..sup.-1) to provide a matching coefficient of thermal expansion. To compensate for the mismatch, only an extremely thin layer of glass such as 0.5.mu. can be used. This in turn requires that the glass be applied by R.F. sputtering techniques, a very limited method of application.
Finally, many glass compositions suffer the disadvantage of having a high softening temperature, that is, a temperature greater than 500.degree. C. Such glasses require special, expensive fixtures to bond them to the metal oxide or metal alloy. The glass discussed above in connection with U.S. Pat. No. 3,458,926 is such a glass.
Thus, there has been a need, prior to this invention, for a glass composition that solves all of the aforesaid problems--that is, has a coefficient of thermal expansion that substantially matches that of the metal alloy or metal oxide to which it is to be bonded, and is resistant to attack by caustic solutions. Most preferably, it also softens at a temperature no greater than 500.degree. C. Glass compositions solving some of these problems are described in, e.g., Japanese Application Kokai No. 81/37246; Russian Pat. No. 382,587; and U.S. Pat. Nos. 3,220,815 and 4,312,951. However, these particular compositions do not solve all these problems. For example, they lack the resistance to caustic solutions that is obtained when at least 5 mole % Al.sub.2 O.sub.3 is included.